1. Field of the Invention
The present invention relates to an exhaust gas purification filter.
2. Description of the Related Art
As a means for purifying the carbon particulates, or the like, emitted from an internal combustion engine for an automobile, etc., a method has been employed in which the particulates are trapped temporarily by a filter and subsequently burned off by heating or using a catalyst.
The filter coated with a catalyst is so configured that a honeycombed structure having a multiplicity of cells makes up an exhaust gas purification filter of which the partitioning walls are coated with the catalyst. The conventional exhaust gas purification filter, as shown in FIGS. 10 and 11, uses a honeycombed structure having cells 90 having alternate ends thereof closed by plugs 95. Specifically, as shown in FIG. 11, the cell end portions in the upstream end surface 91 are alternately closed by the plugs 95 in a checkerwork form, for example. On the downstream end surface 92, on the other hand, the end portions of the cells 90 having the upstream end portions thereof not closed by the plugs 95 are closed with the plugs 95, while the end portions of the cells 90 having the upstream end portions thereof closed with the plugs 95 are left open. Under this condition, the catalyst is carried on the partitioning walls 98.
The use of this exhaust gas purification filter 9 described above makes it possible to trap the particulates on the partitioning walls when the exhaust gas of the internal combustion engine is passed therethrough, which particulates are burnt off, by a catalyst, with the heat of a high-temperature exhaust gas.
The conventional exhaust gas purification filter 9 described above, however, poses the problems described below.
The particulates 88 flowing in the exhaust gas 8 and trapped on the exhaust gas purification filter 9 are not always burnt off at an appropriate time but may be steadily deposited on the partitioning walls 98, as shown in FIG. 12. As long as an exhaust gas 8 low in temperature is being emitted from the internal combustion engine, for example, the burning is not started even by the catalytic action but the particulates 88 only continue to be deposited. In such a case, the pressure loss caused when the exhaust gas 8 passes through a filter, i.e. an exhaust gas purification filter 9 is increased while, at the same time, posing the problem that the filter is eventually heated abnormally and cracked or melted by the combustion heat.
The present invention has been developed in view of the problem points of the prior art described above, and the object thereof is to provide an exhaust gas purification filter capable of suppressing the generation of excessive pressure loss and preventing the melting due to the deposited particulates.
According to a first aspect of the invention, there is provided an exhaust gas purification filter, for trapping the particulates in the exhaust gas, wherein the pressure loss caused when the exhaust gas passes through the exhaust gas purification filter increases until the amount of the particulates deposited in the exhaust gas purification filter reaches a predetermined value, while the pressure loss is not substantially increased after the amount of the particulates deposited exceeds a predetermined value.
In the exhaust gas purification filter according to this invention, as described above, even in the case where the amount of the particulates deposited increases to such an extent that the pressure loss due to the exhaust gas passing through the filter is increased, the pressure loss is not substantially increased after the amount of the particulates deposited reaches a predetermined value. As a result, an excessive pressure loss can be prevented even in the case where the process for burning off the particulates deposited is carried out belatedly and the amount of the particulates deposited exceeds the predetermined value described above. In this way, the adverse effect that the increased pressure loss otherwise might have on the internal combustion engine, etc. can be suppressed.
The exhaust gas purification filter according to this invention may be formed of a honeycombed structure of a metal instead of ceramic.
According to a second aspect of the invention, there is provided an exhaust gas purification filter wherein the predetermined amount of the particulates deposited is preferably smaller than the amount which makes the partitioning wall melt due to the heat generated when burning the deposited particulates. Then, in the case where particulates exceeding a predetermined value are deposited, the trouble of the exhaust gas purification filter being melted can be reduced.
According to a third aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, having a multiplicity of cells surrounded by partitioning walls,
wherein at least some of the cells each have a plug at one of the end portions thereof,
wherein at least some of the plugs to be arranged on the downstream side in the fluid path are partial plugs having an opening allowing the fluid to pass therethrough, and
wherein the pressure loss caused when the fluid entering the cells passes through the partitioning walls is smaller than the pressure loss caused when the fluid passes through the partial plugs.
Now, the operation of this invention will be explained.
The exhaust gas purification filter according to this invention comprises partial plugs each having an opening on the downstream side of the cells. These partial plugs are related to the pressure loss as described above. In other words, the partial plugs are configured to have a larger passage resistance than the partitioning walls against the fluid.
For this reason, the exhaust gas constituting a fluid entering the cells, is not substantially passed through the openings, if any, of the partial plugs but is passed through the partitioning walls and discharged from the adjacent cells having no plugs.
As a result, the particulates in the exhaust gas are trapped by the partitioning walls and appropriately burnt off by the heat of the heater or by the action of the catalyst carried on the partitioning walls of the exhaust gas purification filter.
In the case where the particulates are not readily burnt by the heat of the heater or by the catalytic action, on the other hand, the particulates come to be deposited steadily on the partitioning walls. In such a case, the pressure loss caused when the fluid passes through the partitioning walls increases to a level larger than the pressure loss caused in the partial plugs.
Under this abnormal condition, with the particulates deposited excessively, the fluid that has entered the cells can be discharged from the openings of the partial plugs, and thus the excessive pressure increase under the abnormal condition can be avoided. After the particulates are burnt off by the catalytic action subsequently, therefore, the filter is regenerated into the initial state so that the fluid again begins to flow through the partitioning walls to restart the trapping of the particulates.
With the exhaust gas purification filter according to this aspect of the invention, therefore, an excessive pressure loss caused by the deposited particulates can be suppressed while at the same time making it possible to prevent cracking and fusion loss.
According to a fourth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein each of the end surfaces of the exhaust gas purification filter preferably includes the cells ends having a plug and the cell ends having no plug alternately arranged. This arrangement can readily establish a route along which the exhaust gas passes through the partitioning walls.
According to a fifth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein all the plugs located on the upstream side of the honeycombed structure are preferably complete plugs capable of blocking the passage of the fluid entirely. In the case where the partial plugs are formed on the upstream side surface of the honeycombed structure, the fluid enters directly from the openings of the partial plugs, and the efficiency of trapping the particulates may be deteriorated. By making all the upstream plugs complete, the reduction in the trapping efficiency can be suppressed.
According to a sixth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein the opening rate of the partial plugs in terms of ((Axe2x88x92B)/A)xc3x97100 is preferably in the range of 5 to 80%, where B is the area of the opening of each partial plug and A is the area of the cell opening. In the case where the plug opening rate is less than 5%, the pressure loss caused when the fluid passes through the partial plugs is not increased greatly, while the amount of the fluid directly passing through the filter is increased, thereby probably reducing the efficiency of trapping the particulates. In the case where the plug opening rate is more than 80%, on the other hand, the modification of the complete plugs into the partial plugs may not exhibit a sufficient effect.
According to a seventh aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein the plugs located at the central portion of the downstream end surface of the exhaust gas purification filter are partial plugs, and the plugs located around the partial plugs are complete plugs for completely blocking the passage of the fluid. The flow velocity of the exhaust gas constituting the fluid is higher at the portion nearer to the central portion, so that the pressure increase at the central portion is larger than at the portions surrounding the central portion. As a result, the above-mentioned effects of the partial plugs can be sufficiently exhibited even in the case where only the central portion of the exhaust gas purification filter has the function of avoiding the excessive pressure increase.
According to an eighth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein all the plugs located on the downstream end surface of the exhaust gas filter are the partial plugs. In this case, the excessive pressure increase can be sufficiently avoided regardless of the fluid flow rate distribution.
According to a ninth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein the partial plugs preferably represent at least 30% of all the plugs located on the downstream end surface of the exhaust gas purification filter. In the case where the partial plugs represent less than 30%, the effect of preventing the excessive pressure increase described above may be reduced.
According to a tenth aspect of the invention, there is provided an exhaust gas purification filter of a honeycombed structure, wherein that area of the downstream end surface of the exhaust gas purification filter which is located within the curved line connecting the middle points of the lines connecting the center and the outer periphery of the downstream end surface is defined as a central area, and the area located outside the particular curved line is defined as a outer peripheral area. Then, the partial plugs preferably represent a higher percentage of all the plugs in the central area than in the outer peripheral area. As a result, the effect of avoiding the excessive pressure increase can be exhibited more in the central area than in the outer peripheral area.